Inkjet recording has been known as an excellent recording method in which substantially any recording media (hereinafter, the recording media may also be referred to as “a recording medium”, “inkjet paper”, “recording media for inkjet recording” or “media”) can be applied, and it has been extensively studied and improved in terms of a recording apparatus, recording method, and recording medium thereof. In the art, the widely used ink for inkjet recording is the one which contains water as a main component, and an aqueous dye. The reason for the large demand in such the ink is that this ink uses easily obtainable dyes having high absorption coefficient and high color purity, handling of this ink is very easy when the ink is made in multiple colors for purpose of widening the range of color tones thereof, and the ink can be made so as to have good long-term storage stability and high heat resistance stability, and especially to prevent deterioration or blockage of a thermal head as a result of the ink being suffered from a heat from a heater for a long period.
Conventional inkjet papers, especially gloss media for inkjet printing, are classified into mainly two types, a swollen type and a porous type. Recently, the porous type media have been widely used since they excel in drying speed of the ink. The structure of the typical porous media is such that onto a support, there is provided an ink absorbing layer having pores so as to take the ink therein, and porous glossy layer is optionally provided on the ink absorbing layer. For example, Patent Literatures 1 and 2 disclose a porous medium which is obtained by applying, onto a support, a coating liquid wherein silica and aluminum hydrate are dispersed therein to form one or more layers, and optionally applying a coating liquid containing colloidal silica so as to form a glossy layer containing a large amount of the colloidal silica. This medium is so designed as to match a dye ink which is currently mainly used in inkjet recording, and is widely used as a medium for inkjet printing, especially as a glossy paper. This paper realizes an output image with high gloss and of extremely precise details. However, it also has drawbacks such that raw materials thereof are very expensive, and the production process for this type of paper is very complicated. Therefore, a production cost for the paper is extremely high compared to a glossy coated paper for general commercial printing. For this reason, use of this paper is limited to cases where high quality output images are required, such as outputting of photography or the like, and it is hardly used in the field of the commercial printings which need large numbers of outputting at low cost, such as flyers, catalogs, brochures, and the like. Moreover, there is a demand for improving the ink absorption capability since the numbers of colors of the ink for use are getting larger for obtaining higher quality of images. In order to improve the ink absorption capability, a thickness of an ink accepting layer, i.e. a coated layer, needs to be increased. However, if the thickness is increased, a larger quantity of expensive materials is required. Therefore, there is a problem such that a cost required for producing each medium is increased.
A pigment used in the ink absorbing layer, i.e. the ink accepting layer, needs to have a low refractive index and opacity which allow to maintain high transparency of the ink absorbing layer, and needs to have large oil absorption or specific surface area. Therefore, it is necessary to add a large amount of expensive pigments having a low refractive index and high oil absorption, such as silica and aluminum hydrate, not inexpensive white pigments such as calcium carbonate, kaolin, and the like. When the ink absorbing layer contains pigments having low transparency and high opacity, a coloring material contained in the ink absorbed in the ink absorbing layer is concealed by such pigments, and thus a high coloring density cannot be obtained. In the case where inkjet recording is carried out by using a paper containing such pigments of high opacity together with an ink containing a dye, the color density of the resulted image is corresponding to only the coloring material present in the surface area of the paper even though the amount of the ink to be ejected is increased. Therefore, the resulted image has low color density on the whole, and has low contrast. In the case where the pigment having low oil absorption is used in a paper, the absorption of the ink cannot be carried out sufficiently, and thus bleeding tends to occur.
In order to resolve the above-mentioned problems, Patent Literature 3 proposes to use fine organic particles having a low refractive index so as to realize both the desirable refractive index and whiteness. However, the organic fine particles still lead to high production cost, and thus it is yet difficult to attain a recording medium designed for an ink containing a dye, at low cost.
Moreover, as a design for improving a long-term stability of a printed image, a main stream idea is to penetrate a dye into an ink-accepting layer as deep as possible so as to block influences from air or ultraviolet ray as much as possible, and to protect the dye by using an antioxidant, or a stabilizer which are originally present within the ink accepting layer, since the dye molecule has low resistance to ultraviolet rays and ozone. Therefore, a deep penetration is realized by using a large amount of an ink having a relatively low content of the coloring agent, so as to maintain image reliability. To this end, an amount of the ink necessary for outputting an image is automatically increased. Therefore, down-sizing of a cartridge is difficult to realize, as well as increasing a cost for recording.
Under these circumstances, in inkjet recording, it is still difficult to provide a recording medium for inkjet recording and a recording method which output high quality images at low cost.
In recent years, attentions have been drawn to a pigment ink for inkjet recording. The pigment is insoluble to water, and thus the pigment is generally dispersed in a solvent in the form of fine particles. In view of safety, a pigment ink wherein the pigment is dispersed in water is mainly used in the art. However, such pigment ink has drawbacks as mentioned hereinafter. The aqueous pigment ink tends to cause more aggregations or precipitation of the pigment particles, compared to a dye ink. In order to improve long-term storage stability to the same degree to that of the dye ink, various conditions for dispersion or additives are required. The dispersion stabilizer however becomes a factor for causing a blockage of a thermal head as a result of the ink being suffered from a heat from a heater for a long period. Therefore, such ink is not suitably used with a thermal head. Moreover, the pigment ink is inferiors to a dye ink in a color specification range of coloring agents. Although the pigment ink has these drawbacks, it has been popular in view of storage stability after recording, water-resistance, and image reliability such that high color density of black can be attained. It is considered that an inkjet printer using the pigment ink enables to realize the prints having the improved textures to the level of those of commercial printing, as the coloring agents used therein are close to those of general commercial printing. However, when printing is actually performed on a coated paper for commercial printing by using the conventional pigment ink, drying speed of the ink is not sufficiently high. This poor drying property causes problems such that an image is spread or blurred, the pigment is not fixed at all after drying, abrasion occurs, and the like. Therefore, in reality, the pigment ink can be used only for printing of media having high absorption of an ink, such as a normal paper, a recording paper for inkjet recording, and the like. This is because the designing concept of an inkjet image using the pigment ink has not changed from that of an inkjet image using the dye ink, the pigment coloring agent is considered only as a dye having a high light fastness, and the characteristics of the pigment ink are disregarded.
Moreover, an ink for inkjet recording containing amino acid and the like has been known in the art. However, no attempt has been made to improve a quality of an image printed on a coated paper for commercial printing, by using this ink. For example, Patent Literature 4 discloses an essentially combination of amino acid (including dipeptide) and an amphoteric surfactant as components of an ink and a use of this ink so as to reduce a permeability of the ink into a normal paper and the like for the purpose of improving water-resistance, bleeding, feathering, or the like on a normal paper as a challenge for the dye ink, and also proposes to use amino acid or the like in the ink for the purpose of obtaining excellent optical density.
Patent Literature 5 proposes an ink containing protein or dipeptide, further containing a combination of amino acid and a certain surfactant, for the purpose of improving reliability for preventing clogging of a thermal head. Regarding the reliability for preventing clogging, this literature discloses that the protein or peptide contained in the ink is decomposed due to the structure thereof at the time of using with a thermal head, the decomposed products are deposited on a head heater area resulting in a blockage of a thermal head, and therefore the reliability is significantly lowered. It is further discloses that the above-mentioned lowered reliability for preventing clogging is prevented, and the reliability for preventing clogging is maintained by further adding amino acid and a certain surfactant in combination in the ink.
Patent Literature 6 proposes to add saccharide, reduced saccharide, oxidized saccharide, and the like to an ink for the purpose of improving reliability for preventing clogging, and also discloses that the clogging is prevented by using such the ink as the water evaporation of the ink is inhibited around a nozzle. However, the ink disclosed in this literature is not a pigment ink wherein a moisture content of the ink is reduced and dipeptide is added, so as to realize both of reliability for preventing clogging, and media conveyance and image reliability for preventing beading and cockling, at the time using with commercial media having low absorption of the ink. Here, the term “image reliability” means light fastness, water-resistance and fixing ability of characters or image printed on the media.
Moreover, urea or the like is also listed as a solid moisture retainer in this literature, but comparing to dipeptide, urea has problems such that it tends to slightly increase viscosity of the ink with the same addition amount to that of the peptide, and may increase viscosity of the ink or cause aggregation of a coloring agent due to decomposition thereof ad the time of storage at high temperature.    [Patent literature 1] Japanese Patent Application Laid-Open (JP-A) No. 2005-212327    [Patent literature 2] JP-A No. 11-78225    [Patent literature 3] JP-A No. 2003-25717    [Patent literature 4] JP-A No. 2005-15795    [Patent literature 5] JP-A No. 2006-117634    [Patent literature 6] JP-A No. 2006-122900